The present invention relates in general to a radiation shielding coating composition and a method of making and using it. The invention more particularly relates to compositions and methods for shielding microelectronic devices and other objects and animals, with a radiation-hardened light weight coating for withstanding the radiation hazards found in the space environment, as well as other less hazardous environments.
Many of today""s commercial integrated circuit (IC) devices and multi-chip modules (MCM) cannot be utilized in deep space and earth orbiting applications because of total dose radiation induced damage. Commercial IC devices are developed and manufactured for computer and mass market applications and are not designed to withstand the effects of the natural space environment. The radiation effects include solar flares, galactic cosmic radiation and the Van Allen trapped electron and proton belts or man-made radiation induced events (neutrons and gamma radiation).
Typical commercial silicon integrated circuits fail to operate when exposed to total doses of two to fifteen kilorads(Si). Common methods used to prevent radiation degradation in performance are: 1) to design special radiation tolerant die, 2) to shield the entire component and board assembly, or 3) shield the individual component. There are weight, cost and time-to-market penalties depending on the method. For example, specially designed radiation tolerant die are time consuming and expensive to produce, since the part must be redesigned to incorporate radiation hardening techniques. Examples of such methods include U.S. Pat. Nos. 3,933,530; 4,014,772; 4,148,049; 4,313,7684; 4,402,002; 4,675,978; 4,825,278; 4,833,334; 4,903,108; 5,001,528; 5,006,479; 5,024,965; 5,140,390; 5,220,192; and 5,324,952, each of which patent is incorporated herein by reference. Reference may also be made to Japan patent 62-125651, Jun. 6, 1987, and articles entitled xe2x80x9cEffects of Material and/or Structure on Shielding of Electronic Devices,xe2x80x9d R. Mangeret, T. Carriere, J. Beacour, T. M. Jordan, IEEE 1996; and xe2x80x9cNovice, a Radiation Transport/Shielding Codexe2x80x9d, T. M. Jordan, E.M.P. Consultants Report, January 1960, the Japan patent and such articles being incorporated herein by reference.
Such techniques delay the time to market the products. As a result, these conventional radiation hardened devices are usually two to three generations behind the current commercial technological advances in both size and capabilities. There are additional penalties in limited marketability and demand, and hence low volume productions of the die result. Consequently, such methods produce a more expensive product, which is technologically behind the commercially available microelectronics, with slower speed and less capability. Additionally, because of the limited market for these products, they are frequently not available at all.
Such radiation shielding methods involve using metal shielding external to the package. Shielding by other mechanical or electrical elements complicates the platform design, often requiring complex three dimensional modeling of the design.
Another attempt at shielding includes disposing a small shield on the surface of the package. Such a technique does not provide effective three-dimensional shielding protection. Additionally, the small external shield is generally thermally mismatched to the package, and increases the size and weight of the package.
Examples of system level shielding are disclosed in U.S. Pat. Nos. 4,833,334 and 5,324,952, which are incorporated by reference as if fully set forth herein. The U.S. Pat. No. 4,833,334 discloses the use of a protective box to house sensitive electronic components. The box is partially composed of a high atomic weight material to shield effectively against x-rays. However this approach has the serious disadvantage of adding substantial bulk and weight to electronic circuit assemblies protected in this manner. Moreover, it would be expensive to provide this type of protection to individual integrated circuits as manufacturing custom boxes for each circuit configuration would be costly.
The method of shielding material on the outside of the package is known as spot shielding. Such a technique is disclosed in Japanese patent publication 62-125651, published Jun. 6, 1987, which is incorporated by reference as if fully set forth herein. This patent describes a spot shielded semiconductor device which utilizes a double layered shield film to serve as a sealing cover on an upper surface of a semiconductor package. Another double layered shield film is attached to a lower surface of the package. However, space qualified microelectronic parts must be capable of withstanding the enormous forces exerted during acceleration periods during space travel. The external shields are subject to tearing or prying off from the sealing cover. The use of a double layer shield film only slightly reduces the weight of the package, but increases the size of the package unnecessarily. Also, thin films are generally only effective at shielding electromagnetic interference (EMI) radiation and are ineffective at shielding ionizing radiation found in space. Examples of this type of EMI or EMF shielding devices include devices disclosed in U.S. Pat. Nos. 4,266,239; 4,823,523; and 4,868,716, which are incorporated herein by reference.
The significant disadvantage of the spot shielding method includes an increase in weight and thickness of the device, and an increase in exposure of the semiconductor to side angle radiation due to the shielding being spaced apart from the semiconductor.
A far superior method of shielding involves using an integrated shield, where the package itself is the shield. For example, reference may be made to said U.S. patent application Ser. No. 08/221,506, filed Apr. 1, 1994, entitled xe2x80x9cRADIATION SHIELDING OF INTEGRATED CIRCUITS AND MULTI-CHIP MODULES IN CERAMIC AND METAL PACKAGES,xe2x80x9d which is incorporated herein by reference. The material in the package and the package design is optimized for the natural space radiation environment.
Many conventional microcircuits are only available in prepackaged form, or the die is already mounted onto the circuit board. Therefore, it would be highly desirable to have technique and shielding compositions for shielding parts already packaged or mounted on a circuit board, or in bare IC die form. Such compositions should be relatively inexpensive to manufacture and use, and are compact in size. In this regard, such new and improved techniques should be very convenient to employ in a highly effective manner, and yet be relatively inexpensive to manufacture.
The principal object of the present invention is to provide a new and improved composition and method of radiation shielding in outer space or other environments, whereby such shielding compositions and methods are highly effective and relatively inexpensive.
Another object of the present invention is to provide such a new and improved method and composition, wherein the radiation tolerance of the bare die to be shielded is greatly improved, and the shielding is provided in all axial directions.
A further object of the present invention is to provide such a new and improved method and composition, wherein satellite designers can utilize current generation IC technological advances, while improving delivery time.
A still further object of the present invention is to provide such a new and improved method and composition, wherein IC devices can be supplied relatively inexpensively due to the use of commercially available dies at current market prices without undue weight, excessive or bulky sizes or other undesirable or unwanted design requirements.
Yet another object of the present invention is to provide such a new and improved composition and method of radiation shielding for protecting other objects or animals from unwanted radiation.
Briefly, the above and further objects of the present invention are realized by providing shielding compositions and methods which are relatively inexpensive to use and highly effective in outer space and other environments.
The radiation shielding composition and method of the present invention relate to a conformal coating material composed of a matrix of densely packed radiation shielding particles, which are disbursed within a binder. The shielding composition is applied to objects to be protected such as integrated circuits, or to packages therefor, as well as for protecting animals including humans from unwanted exposure to radiation in outer space or other environments.
The inventive radiation shielding composition including the densely filled conformal coating material is used for commercially available integrated circuits or grouping of circuits, to protect against natural and man-made radiation hazards of the spacecraft environment, whether in earth orbit, geostationary, or deep space probes. The inventive composition and methods are provided to facilitate the design and manufacture of microelectronics, and to coat externally the microelectronics with the inventive shielding composition to improve radiation tolerance to natural space radiation.
The inventive shielding composition, in one form of the invention, includes a fabric and a flexible binder, used to shield animals including humans in space or in other environments. As humans prolong their stay in space, the risks from increased exposure to ionizing radiation become more of a concern. The conventional method of shielding using lead has two major disadvantages. Lead is highly toxic, which is a disadvantage in both manufacture and use. Lead is also relatively less dense. With the inventive composition, the same equivalent shielding can be obtained with a thinner high Z material such as tungsten. By using a denser material, a thinner shield can be constructed, making movement relatively easier. Since sources of radiation are not limited to space, this same material has utility to shield humans or other animals from radiation sources on earth.
The limiting factor is weight, and the energy and species of radiation. Thin densely packed shields are not very effective on high energy electromagnetic radiation such as gamma rays, and high energy neutrons.
Additionally, the inventive conformal coating composition and method are useful as a radiation shielding gasket between enclosures. There are many radiation shielding utilities for the inventive compositions and methods, depending on the choice of the binder material.
The present inventive methods and compositions contemplate using both plastic or ceramic packaged microelectronic devices, as well as unpackaged die and encapsulating or coating the outer surface of the device to provide shielding as required for the anticipated radiation environment. Since fluences of species and energy ranges of radiation vary in space, and since the optimal shielding varies depending on the species of radiation, the coating substance or material can be optimally tailored based on the anticipated radiation that irradiates the part to be protected. In all applications, the particles impregnated within the conformal coating substance are designed to achieve the highest tap density possible for the application.
The present inventive method preferably includes calculating/modeling the anticipated radiation spectrum, the required amount of shielding, as well as multiple layers of both high Z and low Z shielding material. The inventive conformal coating substance or material is then designed to meet that requirement. For a standard Geosynchronous Orbit, the optimum shielding entails a conformal coating having three layers; namely, a high Z layer sandwiched between two low Z layers. For marking and hermiticity, a layer of smooth unimpregnated coating material is applied to the top layer.
For integrated circuit devices that have already been packaged, the inventive conformal coating material can be applied in various manners. These include, but are not restricted to, the following inventive methods. One method relates to using a low pressure (or high pressure depending on the package strength and susceptibility) injection mold. The coating material is injected into a mold containing the packaged part. Another method involves xe2x80x9cglobbingxe2x80x9d or putting a viscous conformal coating over a packaged part. The part can be disposed within a mold, or elsewhere when the shielding composition is applied. Another method involves spraying or painting on the coating composition.
The optimum method is to coat all sides of the part uniformly with the shielding composition to shield all sides equally from isotropic radiation, and especially when the direction of the source of radiation is not known.
For integrated circuits already attached to a board, either in a bare die form or with an existing coating, the coating is applied with a mold, by xe2x80x9cglobbingxe2x80x9d the composition on, by spraying or painting. To shield the top and bottom sides of the die uniformly, the bottom of the board preferably is also shielded with the inventive conformal shielding composition.
For multi-chip modules (MCMs) where there are multiple integrated circuits within a single package, the inventive conformal coating composition is applied in a similar manner as in the monolithic packaged integrated circuit. Similarly, when there are multiple bare integrated circuits, the inventive conformal coating composition is applied in a similar manner as with the single bare integrated circuit, wherein the coating composition is applied to the entire area covered by the devices to be shielded.
For system or boxes containing board level products requiring additional shielding, the inventive conformal coating composition can also be applied to any box or device to be shielded from ionizing radiation. In this manner, with the use of a flexible binder material such as latex, a gasket can be made for sealing two objects, wherein the inventive gasket material also provides a radiation shielding function.
Because of the flexibility of the inventive shielding composition, radiation shielding can be achieved easily and relatively inexpensively for applications that were either previously considered to be excessively expensive or difficult to shield.
For human radiation protection, the inventive composition conformal coating include a latex or similar flexible binder. To enhance the mechanical strength properties, a fabric material is added and combined with the binder. In this form of the invention, a high Z material, which is dense and nontoxic, can be inserted within the layers of clothing material to add extra protection for the wearer from unwanted radiation. Because of weight considerations, the optimal shielding can be obtained in the weightless environment of space. Lighter, thinner material is used for gravity constrained environments. Additionally, the impregnating particles can be tailored for the type of radiation to be encountered, enabling optimal use of space and weight of the material.